Radio frequency (RF) communication systems are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, radios and other devices that receive RF signals. RF receivers typically require frequency translation or mixing. Two-way wireless communication systems include receive path circuitry and transmit path circuitry. In prior solutions, this circuitry has been integrated into a plurality of integrated circuits and included within the system as a wireless communication chip set. With respect to the transmit path circuitry for such systems, intermediate frequency (IF) architectures have been used, where the baseband signal from a digital signal processor (DSP) is first converted to IF and then transmitted within the frequency band of the wireless communication network using offset phase lock loop (offset PLL) circuitry. With respect to cellular phone wireless communication networks, for example, the frequency bands can be around 850 MHz and 900 MHz for GSM cell phone networks, 1800 MHz for DCS cell phone networks, and 1900 MHz for PCS cell phone networks. Channel spacing in these cell phone networks is 200 kHz.
FIG. 2 (prior art) is a block diagram of an example architecture 200 for transmit path circuitry including an IF voltage controlled oscillator (VCO) 202, a transmit (TX) VCO 128 and an RF VCO 204. When the device is transmitting, I and Q baseband signals 107 are received from a digital signal processor (DSP) by the IF mix circuitry 220. The IF mix circuitry utilizes the IF VCO 202 to convert the I and Q signals to IF. The output 112 of IF mix circuitry 220 then goes to offset loop feed-forward circuitry 122. This offset loop feed-forward circuitry 122 will compare the phases of the signal 112 from the IF mix circuitry 220 with a signal 114 from the feedback mix circuitry 224. The offset loop feed-forward circuitry 122 will then use the phase difference between these signals to tune the TX VCO 128 to produce an output signal (OUTPUT) 105 at a desired frequency of a transmit channel within the frequency band for the communication network. The feedback mix circuitry 224 receives the output signal 105 and mixes the frequency down using the RF VCO 204 and a channel signal (CHANNEL) 127. The output signal 114 is then provided as feedback to the offset loop feed-forward circuitry 122. One problem with this prior architecture, where integration of the transmit path circuitry is attempted, is interference among the IF VCO 202, the TX VCO 128 and the RF VCO 204. In particular, spurs caused by the IF VCO 202 can be problematic and can significantly degrade performance, particularly when they fall within receive bands for GSM cellular communication networks.
One prior solution to this IF VCO 202 interference problem was to adjust the IF for certain troublesome receive channels. In other words, for a first set of transmit channels, the IF VCO 202 would be tuned to a first transmit IF. And for a second set of transmit channels, the IF VCO 202 would be tuned to a different IF. In this way, troublesome interference could be reduced by simply moving the position of the potential interfering frequencies caused by the IF VCO 202. This solution, however, is cumbersome in that it requires an IF VCO 202 that must be adjusted to multiple IFs. And this solution still utilizes both an RF VCO 204 and an IF VCO 202.